LXIV INTRODUCTORY REMARKS. 



formation spreads out over an extensive space south of the Black Hills in 

 Nebraska, and extends into Colorado and the northwestern corner of Kansas, 

 narrowing rapidly southward, however, after passing into those States. A few 

 vertebrate remains are sometimes found in it south of the Bad Lands; but 

 the latter seem to be the grand repository of these interesting relics of 

 extinct beings. 



On Loup River, a small tributary of the Platte, and at other places in 

 Nebraska, there is a Pliocene lacustrine deposit overlying the White River 

 group, or rather occupying depressions in it, that has also afforded some 

 interesting extinct vertebrate remains, some years back described by Dr. 

 Leidy, and others by Professor Marsh. As these beds, however, have only 

 afforded a few species of invertebrates (Helix, Physa, etc.), and these ap- 

 parently of recent species, this formation need receive no especial notice here. 



Before closing this introduction the author desires to acknowledge his 

 obligations to Capt. H. T. Brian, the efficient foreman of printing at the 

 Government Printing-Office, for his efforts to reduce to a minimum the diffi- 

 culties arising from the proof-sheets having to be sent to distant parts of the 

 country for correction as the work went through press. 



